The present invention relates to a method of forming integral internal channels in glass tubing and to the resulting articles of manufacture. More particularly the invention relates to a method for forming such channels wherein there is a continuous fusion band formed by fusing a smaller tube along the length of the inner walls of a larger tube.
The nature, composition and treatment of glass is described in Kirk-Othmer, Encyclopedia of Chemical Technology, Volume 10, pages 533 to 604 (1966, 2nd Edition). The silica (quartz) and borosilicate glasses discussed in this publication are of particular interest in relation to the present invention since they can be softened to make them plastic but not fluid and thus can be worked at elevated temperatures. The other conventional glasses have too low a fluid temperature and/or are otherwise unsuitable for the present invention. This kind of low softening temperature glass is shown in U.S. Pat. No. 3,679,385, for instance.
The strain point of glass is defined as that temperature at which the internal stresses in the glass are reduced in a matter of hours depending upon the mass of the glass article. The viscosity of the glass at the strain point is usually about 10.sup.14.5 poises. The annealing point of glass is defined as that temperature at which the internal stresses are reduced to acceptable limits for commercial use of the glass in a matter of minutes. The viscosity at the annealing point is about 10.sup.13 poises. The borosilicate glasses have a strain point temperature above about 400.degree. C, an annealing temperature above about 500.degree. C and a softening point slightly higher. Quartz has a strain point temperature above about 800.degree.C, an annealing point above about 900.degree. C and a softening point of about 1500.degree. C. The lower strain point and higher annealing temperatures are a function of the composition of the glass. Such glasses are sold in clear form, but can be surface colored in ways well known to those skilled in the art.
The formation of glass to glass seals in high softening temperature silica and borosilicate glasses and heat treatment of glass tubes is well known and the prior art is illustrated in U.S. Pat. No.'s 3,320,352; 3,536,462 and 3,634,056. These patents disclose that ring seals can be formed by heating a small section of a larger diameter tubing in mating contact with the outside surface of a smaller diameter tubing while applying a reduced pressure inside the outer tube. In this manner the ring seal closes and bonds the ends of the tubes so joined. However, those skilled in the art have believed, when long spaced sections of glass are fused in this manner, that the internal strains resulting upon cooling of the glass would cause cracking and breakage even with annealing. Thus, glassware with integral internal channels for applications such as heat transfer have been unavailable as articles of commerce.
The prior art has produced tubular glass structures with internal glass channels along their length spaced away from the tube walls for heat transfer applications, particularly reflux and still-head condensers to be used for laboratory bench scale chemical reactions. Such condenser structures would be more useful if they could provide more efficient heat transfer and could be stronger. These novel structures would have a shorter length for equivalent heat transfer by comparison to prior art structures.
It is therefor an object of the present invention to provide a novel method whereby integral internal channels can be formed along substantial inside lengths of glass tubing without leaving strains in the glass which cause cracking and breakage upon cooling. It is further an object of the present invention to provide novel articles of manufacture comprising glass tubes with self-contained integral internal channels particularly useful for heat transfer applications by this method which are strong and provide relatively more efficient heat transfer in glass. Typical heat transfer applications include the condensation of vapor in liquid distillation condenser units, but the articles are also useful for the cooling or heating of liquid or gaseous streams as will be apparent to those skilled in the art. Further, coils of electrical resistance wire can be provided in the internal channels to provide heating surfaces in a cylindrical vessel for the heating of liquids. Vessels of such a unique design would have great utility for conducting chemical reactions and would be superior to oil baths and electric heating mantles now employed for this purpose in chemical laboratories and pilot plants.